This invention relates to the continuous or intermittent removal of particulate polymer product from a fluidized bed system operating in the condensing mode, that is, where liquid is added, recycled and condensed outside the reactor to enhance the removal of the heat of reaction.
The widely used xe2x80x9cUnipolxe2x80x9d fluidized bed olefin polymerization process employs two tanks in series for the intermittent removal of granular polymer from the fluidized bed, generally as is illustrated in Aronson""s U.S. Pat. No. 4,621,952. In the original design for use with a dry gas phase reaction system, the discharge nozzle was located near the bottom of the fluidized bed. Recent advancements in heat removal and reactor static control have led to partial condensation of the reactor feed gas. The two-phase vapor and liquid mixture enters the reactor fluidized bed from the bottom. Thus significant quantities of liquid may exist at and near the bottom of the fluidized bed. The inlet fluid is ultimately completely vaporized, absorbing the heat of reaction as it moves upward through the polymerization zones of the fluidized bed reactor. But unfortunately during a discharge event, liquid may be carried out of the reactor along with the granular polymer when it is removed for further processing and shipment. Significant cooling can occur within the product discharge tanks as the liquid vaporizes.
Vaporization of liquid in the product discharge system may cause the pressure in the blow tank and other parts of the system to approach dangerous and/or maximum allowed working pressures, as the original design for the equipment for a dry system did not anticipate such vaporization. Particularly in the case of polypropylene production, this can be a limiting production rate factor.
The pressure in the product discharge tanks is affected particularly by flashing of the liquid from the product as soon as it enters the relatively low pressure product discharge tank. In addition, any liquid which remains in the liquid phase takes up volume in the tank which could be occupied by resin product, and tends to reduce flowability of the resin. The upward flow of gas vaporizing from liquid originally settling in the bottom of the tank provides resistance to the downward flow of the granular product, thus also retarding the introduction of the product to the tank.
Low temperatures may occur in the product discharge tank or the blow tank as the liquid evaporates, especially when the condensing agent is propylene or propane.
The liquid that escapes with the resin out of the reactor must be recovered into the reaction system to achieve economical use of feedstocks. Commercial systems are designed with elaborate monomer and feedstock recovery schemes for this purpose. One of these is the Unipol improved product discharge system, sometimes called the IPDS. Regardless of the recovery scheme, it is clearly advantageous to reduce the amount of liquid that escapes with the resin. If the amount of liquid reaching the product discharge tank is minimized, the size and costs of the recovery equipment can be reduced and less feedstock may be lost.
The liquid""s flash may also reduce the resin""s temperature in the product discharge system. This is undesirable where the solubility of monomer and/or condensing agent in the product is enhanced by lower temperatures.
Liquid may also contribute to progressive increases in the baseline pressure of a product discharge system over multiple resin discharge cycles. Baseline pressure is that pressure in the product discharge tank when the discharge valve first opens or the pressure in an empty tank as the system cross-ties. An increase in baseline pressure reduces monomer savings effected by a cross-tie to another tank.
Generally, the prior art has not dealt with the above problems caused by the ever-increasing quantities of liquid present in the system and liable to be removed from the reactor with product. Patents disclosing product discharge configurations after the above mentioned Aronson patent 4,621,952 have not addressed minimizing the amount of liquid in the product. See, for example, DeLorenzo U.S. Pat. No. 4,535,134, which employs weirs in a horizontal reactor. The weirs tend to assure that the product removed has already settled at levels higher than half the height of the reactor, but in the end the product drain used at the end of the reactor is at its bottom. The apparatus of EP 0 830 892 A1 removes product from the top of the fluidized bed primarily by gravity in order to minimize the amount of gas removed with the product.
The sloped discharge (of EP 0 890 892) is said to be from 0.6 to 0.95H. This allows sufficient height that the discharge system tanks and associated hardware can be built at ground level when the reactor vessel is essentially located at ground level. The 0.6H to 0.95H specification also accommodates the height required in the construction of the sloped chute from the reactor port to the resin receiving vessel. The removal of product high in the reactor also aids in the removal of resin fines that are prone to accumulate near or at the top of the fluidized bed. That is particularly a problem when operating with a cyclone separator in the gas recirculation line from the top of the reactor to the bottom. Fines in the recirculating gas are separated and returned to the top of the fluid bed by the cyclone. See Bontemps et al U.S. Pat. No. 5,382,638.
As indicated in the above mentioned Aronson patent 4,621,952, product may be removed from the reactor to the product discharge tank by utilizing the difference in pressure between the upper and lower ports of the fluidized bed. While there is an initial rush of polymer particles into the discharge vessel when the discharge valve is opened due to gravity and the pressure difference between the reactor and the discharge vessel, this is ineffective in filling the vessel to near its capacity within a practical time limit. To increase the rate of product discharge after the initial rush, a vent is opened to the upper regions of the reactor to take advantage of the pressure difference, typically from 2 to 12 psi, between the higher pressure low regions and the low pressure upper regions of the bed. Some of the upward flowing fluidizing gas is caused to leave the bed and pass through the discharge vessel to reach the upper region of the bed, and in so doing, product is conveyed from the reactor into the discharge vessel. This greatly increases the amount of product removed during each discharge cycle.
While we do not purport to coin any new terms, it may be useful to discuss the meanings of a few terms used in this application, as there may be some disagreement among practitioners of the art as to their meanings. For example, we have construed the letter xe2x80x9cHxe2x80x9d as used in EP 0 830 892 A1 to mean the height of the reactor wall around the fluidized bed as illustrated in that patentxe2x80x94in other words the height of the fluidized bed as defined by the straight or cylindrical wall only, beginning with the distributor plate, terminating at the top of the straight or cylindrical wall, and not including the expanded zone above the straight or cylindrical wall or any conical section such as is commonly used to transition from the straight (cylindrical) section to the expanded section. This is the meaning used in the present application. When we state that a port in the reactor wall is located at a certain distance from the distributor plate, such as 0.15H, we mean that the center of the port is at that vertical distance from the distributor plate. Also, the term xe2x80x9ccondensing modexe2x80x9d is used to include a fluidized bed process in which fluid from the reactor is removed, cooled and condensed to remove the heat of reaction, and returned to the reactor. The fluid may contain from 1% to 95% liquid by weight after cooling and condensing, and may or may not contain non-reactive materials added to enhance the efficiency of heat removal. Thus, as an example, where 20% by weight of the recycled fluid entering the reactor after cooling and condensing is liquid, the operation may be referred to as xe2x80x9c20% condensing.xe2x80x9d
The concentration of liquid varies with height in the bed during condensing operation from most concentrated at about the distributor plate to the lowest concentration or no liquid in the cycle gas at the top of the fluid bed. This may be referred to as the extent of condensing penetration into the bed, the extent of condensing or the liquid or condensing gradient. It is also recognized that the concentration of liquid may vary radially in the bed, and the liquid gradient is meant to represent a composite average at a given height. The liquid may all vaporize within a relatively short distance above the distributor plate, such that the liquid concentration goes from its maximum concentration at about the plate to essentially zero. It is optimal that there be no liquid at the height of the resin discharge port to the product chamber (the product discharge tank), but it should be noted that some liquid can still be present and the performance of the product chamber (product discharge tank) may not be greatly affected.
The extent of condensing, or recycle liquid, penetration into the bed varies with many factors. Most obviously, the higher the condensing level, the greater the penetration. An increase in the cycle gas velocity at constant condensing level may increase the penetration of liquid into the bed during condensing mode operation, and can be detected using thermocouples inserted into the bed or along the internal wall of the reactor, as the vaporization of the liquid reduces temperatures. Increased liquid penetration can have a quieting effect on wall thermocouples in regions the liquid reaches. Electrical charge phenomena in the fluid bed are also mitigated by increased liquid penetration. An increase in cycle gas velocity at constant production rate during condensing mode operation may also increase liquid penetration even though the amount of condensing is less.
Condensing penetration is also affected by the design of the distributor plate, the hole size, the number of holes, the jet penetration at the plate, the pressure drop across the plate, the design of the cap over the hole, the design of the cycle line manifold below the distributor plate, the method of liquid introduction, i.e. whether single fluid or two fluid nozzles are used, the average particle size and particle size distribution of the liquid droplet in the gas, the average size and size distribution of the polymer particles in the fluid bed, and the type of fluidization and its quality.
Condensing penetration is also affected in a complex relationship between the choice of condensing agent and how close the cycle gas dew point approaches the resin bed temperature. The manipulation of the dew point of the recycle fluid with respect to the resin bed temperature is also called the dew point approach temperature. It may be adjusted by selection of condensing agents, condensing agent concentration, total pressure, composition and concentrations of the cycle gas and the temperature of the fluidized bed.
This invention is a method of discharging granular or particulate product from an exothermic fluidized bed polymerization reaction system operating in the condensing modexe2x80x94that is, a fluidized bed system wherein fluid is removed from the reactor, cooled and/or condensed to remove the heat of reaction, and returned to the reactor. While the liquid in the fluid recycle is beneficial for the removal of the heat of reaction, thereby permitting enhanced production rates, liquid in the granular product is undesirable for a number of reasons mentioned above. We have determined that the quantity of liquid removed with the product during operation in the condensing mode can be minimized, without sacrificing efficiency of product removal by pressure difference, by removing product from bed levels of 0.21H to 0.59H. Product removal may be continuous or intermittent and may be conducted entirely by gas transfer, that is, by pressure difference (the difference in pressure between the reactor at the point of discharge and the product receptacle, referred to herein as the product discharge tank), but must be accompanied by appropriate venting of the product discharge tank to a low pressure location in the reaction system. The product discharge tank may be vented before, during or after the discharge valve is opened to allow product to enter the tank. In one method, the tank is vented about 1 to 60 seconds prior to opening the discharge valve, preferably about 5 to 15 seconds before opening the valve. In another method, the tank is vented at about the same time that the discharge valve is opened, recognizing that a few seconds may be required for the valve action to completely open the valve. In another method, the tank is vented 1 to 100 seconds after the discharge valve is opened, The gas in the product discharge tank is preferably vented to a point in the reactor that is above 0.6H, preferably above 0.8H. The vented gas from the product discharge tank may be sent to the fluidized bed, or to the transition or expanded section above the bed. The vented gas may also be sent to a lower pressure point in the gas recirculation loop such as at the suction side of the recirculation gas blower or downstream of a heat exchanger. A blower may also be employed on the vent line to return the gas from the product tank to a location in the reactor higher in pressure than that in the product tank. The gas sent to the reactor comprises both the gas originally in the product discharge tank and that which may be carried into it by the product discharge processxe2x80x94that is, by the flow of product and fluid from the reactor to the product discharge tank caused by the difference in pressure between them. The gas returned to the reactor will therefore include also gas which is vaporized from liquid carried with the product and transfer fluid from the reactor; such liquid may possibly accumulate somewhat in the product discharge tank before being vaporized. The returned gas includes significant amounts of unreacted monomer and therefore represents an economic benefit. Our invention is applicable to the condensing mode operated to any degreexe2x80x94that is, where the recycle fluid comprises anywhere from 1.0% to 95% liquid by weight or more, especially from about 2 to about 50 weight percent liquid.
It is necessary in our invention to continue to provide for a discharge site near the bottom of the bed, in order efficiently to empty the reactor and to provide for the removal of larger particles and agglomerates, sometimes called rubble, that may accumulate near the bottom of the bed. The lower discharge port may be below the distributor plate, just above it (0.002H to 0.15H), or, less desirably, at a level up to 0.2H; there may be more than one lower discharge port. It should be noted here that our references to the location of the lower discharge port and other ports in the fluidized bed are measured to the center of the port; both the lower and upper discharge ports are typically six to twelve inches in diameter. The lower discharge port may be operated intermittently as needed to remove rubble, or periodically, such as at the same time as the upper one described above if it is operated periodically during steady state operation of the reactor. It may be operated at varying rates, but in any case with the understanding that rubble removed from such a low level is highly likely to contain more liquid than product removed from the higher level. Rubble is not necessarily waste, but frequently will require some kind of special treatment if it is to be recycled or otherwise used. The lower product discharge is, however, normally conducted at times other than when the higher withdrawal is conducted. It may be conducted to the same product discharge tank as described above, and at the same time that the higher discharge process is conducted, or to a separate product discharge tank, other vessel or conveyor or blow tank, and/or at different times. It may be observed that one of the advantages of the present process is that it is not necessary for the primary product discharge tank to be lower than the reactor; the receptacle for product from the lowest levels of the bed generally will be lower than the reactor, sometimes necessitating an excavation below the pad for the reactor and/or requiring construction of an elevated platform or support for the reactor significantly above ground level.
The invention also includes the concepts of more than one product discharge tank and of a further tank below the initial product discharge tank, sometimes known as a blow tank, all as contemplated in the Aronson ""952 patent. As is known in the art, the blow tank receives granular material from the product discharge tank and provides a separate zone for further preserving monomer. We also contemplate reducing the pressure in the product discharge tanks by cross tying them as also described in the Aronson patent.
Thus our invention includes a method of removing granular product from a fluidized bed polymerization reactor having a fluidized bed height H and operated in the condensing mode, comprising continuously or intermittently removing granular product from the reactor at least partially by gas transfer, at a level 0.02H to 0.8H preferably at a level 0.21H to 0.59H, to a product discharge tank vented to the reactor at a bed level of at least 0.6H, and providing for rubble removal at a lower level.